The image on the television screen consists of pixels, arranged horizontally in rows, generally offset vertically by one pixel position from one another. Each pixel is assigned three values, which indicate the respective intensities of the red, green, and blue components of the pixel. A video image is generated by sequentially displaying the rows of pixels as horizontal lines of the image.
Existing analog broadcast standards such as NTSC, PAL and SECAM use two video fields to generate a single video frame. Each field includes one-half of the horizontal lines that make up the image frame. One field includes all of the odd numbered lines of the frame and the other field includes all of the even numbered lines. Interlaced images exhibit distorting artifacts such as image flickering that degrade the resulting frame image. One way to eliminate these artifacts is to convert the interlace-scanned fields into progressive-scan frames. In a progressive-scan frame, both the odd and even image lines are displayed sequentially as a single image.
Interlace-scan to progressive-scan conversion systems are gaining importance as more television viewers purchase high-definition television monitors that can display progressive-scan signals. Both broadcast facilities and consumers may want to have interlace-scan to progressive-scan conversion capability in order to avoid the distorting artifacts of interlace-scan images.
One way to generate progressive-scan frames from interlace-scan fields is to interpolate interstitial lines in each field. Thus, the lines of the odd field are used to interpolate even-numbered lines and the lines of the even field are used to interpolate odd-numbered lines.
Each pixel of the interpolated line (or the “interpolated pixel”) is calculated based on the values of proximate pixels in adjacent interlace-scan lines. The simplest method of generating the interpolated pixel is simply duplicating the pixel from the corresponding position in the previously received scan line. For pixels which lie on a diagonal edge, this could result in “jaggies” (a line which appears to be jagged or stair-stepped, rather than smooth). For pixels which are not on an edge, such duplication could result in pixels that do not correspond to the image being displayed, resulting in a poor display to the viewer. This method also reduces the vertical resolution of the image compared to an interlace-scan image and may result in areas of the image flickering at a 30 Hz rate.
Another simple method is to set the value of the interpolated pixel as being the average of two vertically adjacent pixels. However, for a pixel on the edge of two visually distinct regions, such an averaging could result in a pixel that matches neither adjacent pixel. For example, the value generated for an interpolated pixel between a blue pixel and green pixel may be cyan, which would not result in the image desired to be presented to the viewer.
FIG. 5 shows an image 100 on a television screen, consisting of two visually distinct regions 102 and 104. The border 106 between the two visually distinct regions is referred to herein as an edge. An image on a television screen may consist of more than two visually distinct regions, and any one or more visually distinct regions may not be entirely contained within the television screen, as is illustrated.
Visually distinct regions are defined by the edge between them, in contrast to a more gradual change, such as a shadow (which may have gradations of gray and black) or light on a wall (which may have gradations of color). In generating an interpolated pixel which is to be on an edge, it is desirable to consider the visual smoothness of the edge being displayed. If the value of the pixel being interpolated were based solely on the pixels proximate in the received scan lines, the calculated value may be a blend of the values of the two visually distinct regions, rather than a distinctive edge separating the two regions. The result could be an edge without sufficient clarity to distinguish between the two regions, a line that is not visually smooth, or a pixel that has the correct value for an edge but is displayed at the wrong pixel location. Therefore, pixels of an interpolated line which lie on an edge between two visually distinct regions desirably take into consideration not only the values of the pixels proximate in the received scan lines, but also the edge itself, to ensure as visually pleasing an edge as possible.
Prior interlace-scan to progressive-scan conversion systems have recognized this problem and have processed pixels on edges differently than pixels in the regions that are separated by the edges. One such system is described in U.S. Pat. No. 5,886,745 entitled PROGRESSIVE SCANNING CONVERSION APPARATUS issued Mar. 23, 1999, to Muraji et. al., the contents of which are incorporated herein by reference for its teaching on interlace-scan to progressive-scan conversion systems.
The above-referenced patent to Muraji et al. calculates the angle of the edge in a local region based on pixel gradients in the region. This angle is then used to identify appropriate pixels to be used to generate interpolated pixels. While such calculations yield very precise results, there is a cost in speed, memory usage, and overhead.
In addition, existing gradient operators do not specifically address problems inherent with interlaced video signals in which adjacent lines from the image frame are missing, nor do they address the issue of noise in the input signal.